In systems for viewing content such as video content and music content, there is known technology that measures the position of the viewer (user), and based on the measured position of the user, conducts acoustic correction on a music signal so that a more suitable sound field is reproduced in the user's viewing environment. For example, Patent Literature 1 discloses an audio set that picks up a measurement sound output from multiple speakers while varying the position of a pair of microphones, and measures the relative position between the speakers and the pair of microphones based on the picked-up signal. As another example, Patent Literature 2 discloses an audio-visual (AV) system that emits an ultrasonic wave from at least one of multiple speakers, and detects a user based on changes in the echo pattern of the received ultrasonic wave.